Conventionally, various filtering apparatuses are used for filtering water such as seawater, lake water, river water, drinking water or sewage water, for filtering liquids widely used in industries such as cooling water or process liquids for any of various apparatuses, and for filtering ingredient gases or the like used in chemical factories or the like, so as to trap and remove substances such as microparticles and dust included in such fluid to be filtered.
When the filtering apparatus performs filtration for long periods, solid and/or gel-like substances such as dust trapped by the filter elements are accumulated in the filter elements. This increases the resistance of the filter media for the fluid to pass through, and eventually makes the filtering apparatus hardly capable of filtering the fluid. To address this problem, the filtering performance of the filter elements is restored by, for example, periodically performing an operation called “backwashing,” which is a process of causing fluid to flow through the filter elements in the reverse directions of those for filtration so as to release objects trapped and adhered to the filter elements.
This backwashing operation is effective but sometimes fails to completely remove the objects that are trapped and adhered to the filter elements, so that some objects might be left trapped in the filter elements after backwashing. Accordingly, even repeating of the backwashing operation might not prevent an increase in the resistance of the filter media to the fluid flow, which eventually makes the filtering apparatus hardly capable of filtering the fluid. In particular, when gel-like substances such as dust and/or dust coated with highly adhesive materials are strongly stuck on the surfaces of the filter elements, as in the case of filtering the industrial liquid, it is difficult to restore the filtering performance of the filter elements only by simple backwashing of causing fluid to flow through the filter elements in the reverse directions of those for filtration. This is especially true in a filtering apparatus including filter elements of a mesh size less than 200 μm.
Much the same is true for filtration performed under the condition where substances such as fibrous dust are entangled in the filter elements. Such a filtration condition is very likely to occur especially when the fluid to be filtered includes microorganisms, as in filtering water such as seawater, lake water, river water, drinking water or sewage water. For example, when filtering ballast water (seawater) in a vessel's ballast tank, the filtering apparatus traps therein a lot of algae included in the seawater. As a result, fibrous substances including fibrous algae and/or cellulose substances are entangled in the filter elements and cannot be easily removed therefrom.
To address the above, a filtering apparatus capable of restoring the filtering performance of the filter elements has been proposed. An example of the filtering apparatuses of this type includes an inlet channel for the flow to be filtered, an outlet channel for the filtered flow, a number of parallel filtering elements and at least one washing mechanism. The filtering elements are open from both ends, into which the flow to be filtered may be directed so that the infiltration occurs through the element out of the element. The washing mechanism connects alternately to the different ends of the elements at both ends of the filtering elements, and forms a discharge channel for the backflushing of the elements generated with the pressure of the filtered flow. Each of the filtering elements is divided in two between its ends so that the washing mechanism connected to the end of the element generates backflushing only for a part of the element length at a time (see Patent Document 1, for example).
To address the above problems, the present inventors have also proposed a filtering apparatus, which includes, in the interior of the filter element, a trapped object remover configured to be movable in the axial direction of the filter element by the fluid flow generated during filtration or backwashing while its outer peripheral member slides in contact with the inner periphery of the filter element so as to remove trapped objects (See Patent Document 2).